Since the demonstration of low operating voltages by Tang et al., 1987 [C. W. Tang et al. Appl. Phys. Lett. 51 (12) 913 (1987)], organic light-emitting diodes have been promising candidates for the realization of large-area displays. They consist of a sequence of thin (typically 1 nm to 1 μm) layers of organic materials, which preferably are produced by vacuum deposition or by spin-on deposition in their polymer form. After electrical contacting by metallic layers they form a great variety of electronic or optoelectronic components, such as for example diodes, light-emitting diodes, photodiodes and transistors, which, in terms of properties, compete with established components based on inorganic layers.
In the case of organic light-emitting diodes (OLEDs), light is produced and emitted by the light-emitting diode by the injection of charge carriers (electrons from one side, holes from the other) from the contacts into adjacent organic layers as a result of an externally applied voltage, subsequent formation of excitons (electron-hole pairs) in an active zone, and radiant recombination of these excitons.
The advantage of such organic components as compared with conventional inorganic components (semiconductors such as silicon, gallium arsenide) is that it is possible to produce large-area elements, i.e., large display elements (visual displays, screens). Organic starting materials, as compared with inorganic materials, are relatively inexpensive (less expenditure of material and energy). Moreover, these materials, because of their low processing temperature as compared with inorganic materials, can be deposited on flexible substrates, which opens up a whole series of new applications in display and illuminating engineering.
The basic construction of such a component includes an arrangement of one or more of the following layers:                1. Carrier, substrate        2. Base electrode, hole-injecting (positive pole), usually transparent        3. Hole-injecting layer        4. Hole-transporting layer (HTL)        5. Light-emitting layer (EL)        6. Electron-transporting layer (ETL)        7. Electron-injecting layer        8. Cover electrode, usually a metal with low work function, electron-injecting (negative pole)        9. Encapsulation, to shut out ambient influences.While the foregoing represent the most typical case, often several layers may be (with the exception of the 2nd, 5th and 8th layers) omitted, or else one layer may combine several properties.        
U.S. Pat. No. 5,093,698 discloses that the hole-conducting and/or the electron-conducting layer may be doped with other organic molecules, in order to increase their conductivity. Further research, however, has failed to advance this approach.
Additional known approaches for the improvement of electrical properties of OLEDs (i.e., especially operating voltage and light-emission efficiency) include:                1) Improving the light-emitting layer (novel materials) [Hsieh et al., U.S. Pat. No. 5,674,635];        2) Constructing the light-emitting layer from a matrix material and a dopant, where transfer of energy takes place from the matrix to the dopant and radiant recombination of excitons takes place only on the dopant [Tang et al., U.S. Pat. No. 4,769,292, U.S. Pat. No. 5,409,783, H. Vestweber, W. Riess: “Highly efficient and stable organic light-emitting diodes,” in Synthetic Metal 91 (1997), pp. 181–185];        3) Producing polymers (capable of spin-on deposition) or substances of low molecular weight (capable of vacuum deposition) which combine a number of favorable properties (conductivity, layer formation), or producing them of a mixture of a variety of materials (especially in the case of polymer layers) [Mori et al., U.S. Pat. No. 5,281,489];        4) Improving the injection of charge carriers into organic layers by using a number of layers with stepwise coordination of their energy levels, or using appropriate mixtures of a number of substances [Fujii et al., U.S. Pat. No. 5,674,597, U.S. Pat. No. 5,601,903, Sato et al., U.S. Pat. No. 5,247,226, Tominaga et al., Appl. Phys. Lett. 70 (6), 762 (1997), Egusa et al., U.S. Pat. No. 5,674,597];        5) Improving the transport properties of transport layers by admixing a more suitable material with the transport layer. There, transport takes place in for example the hole layer on the dopant/the admixture (in contrast to the doping mentioned above, in which transport of the charge carriers takes place on the molecules of the matrix material) [Y. Hamada et al., EP 961,330 A2].        
Unlike light-emitting diodes based on inorganic materials, which have long found wide use in practice, organic components have hitherto had to be operated at considerably higher voltages. The reason for this is believed to be due to the poor injection of charge carriers from the contacts into organic layers and in the comparatively poor conductivity and mobility of charge-carrier transport layers. A potential barrier is formed at the contact material/charge-carrier transport layer interface, which makes for a considerable increase in operating voltage. The use of contact materials with a fairly high energy level (=low work function) for the injection of electrons into the adjacent organic layer, as is shown schematically in U.S. Pat. No. 5,093,698, or contact materials with still lower energy levels (higher work functions) for the injection of holes into an adjacent organic layer, might provide a remedy. However, the extreme instability and reactivity of the corresponding metals, and the low transparency of these contact materials undercut this potential remedy. In practice, therefore, at this time indium tin oxide (ITO) is used almost exclusively as the injection contact for holes (a transparent degenerate semiconductor), whose work function, however, is still too limited. Materials such as aluminum (Al), Al in combination with a thin layer of lithium fluoride (LiF), magnesium (Mg), calcium (Ca) or a mixed layer of Mg and silver (Ag) may be used for the injection of electrons.
The use of doped charge-carrier transport layers (p-doping of the HTL by admixture of acceptor-like molecules, n-doping of the ETL by admixture of donor-like molecules) is described in U.S. Pat. No. 5,093,698. Doping in this sense means that the admixture of doping substances into the layer increases the equilibrium charge-carrier concentration in this layer, compared with the pure layers of one of the two substances concerned, which results in improved conductivity and better charge-carrier injection from the adjacent contact layers into this mixed layer. The transport of charge carriers still takes place on the matrix molecules. According to U.S. Pat. No. 5,093,698, the doped layers are used as injection layers at the interface to the contact materials, the light-emitting layer being found in between (or, when only one doped layer is used, next to the other contact). Equilibrium charge-carrier thickness, increased by doping, and associated band bending, facilitate charge-carrier injection. The energy levels of the organic layers (HOMO=highest occupied molecular orbital or highest energetic valence band energy; LUMO=lowest unoccupied molecular orbital or lowest energetic conduction band energy), according to U.S. Pat. No. 5,093,698, should be obtained so that electrons in the ETL as well as holes in the HTL can be injected into the EL (emitting layer) without further barriers, which requires very high ionization energy of the HTL material and very low electron affinity of the ETL material. However, such materials are hard to dope, since extremely strong acceptors and donors would be required, so that these conditions cannot be fully met on both sides with materials that are actually available. Now, if HTL and ETL materials that do not meet these conditions are used, when voltage is applied an accumulation of charge carriers develops in the transport layers at the interfaces to the EL. Such an accumulation in principle promotes non-radiant recombination of excitons at the interface by, for example, the formation of exciplexes (these consist of a charge carrier in the HTL and ETL, and of the opposite charge carrier in the EL). Such exciplexes recombine mainly non-radiantly, so that exciplex formation represents a non-radiant recombination mechanism. In addition, the problem of exciplex formation becomes more acute when doped HTLs and ETLs are used, since in doped materials the Debye shielding length is very small and hence high charge-carrier thicknesses occur directly at the interface. In addition, dopants in the immediate vicinity of the EL may result in quenching of fluorescence by, for example, Förster transfer.
Blocking layers in OLEDs for improving the charge carrier balance in the respective light-emitting layer are disclosed in the literature. Their function consists of preventing charge carriers from leaving the light-emitting layer. In the case of electrons in the emitter layer, therefore, the condition is that the LUMO of the electron blocking layer (which is located between the emitter and the hole-transport layers) must lie distinctly over the LUMO of the emitter layer, and the blocking layer must be designed thick enough so that no tunneling of electrons into the following hole-transport layer can take place. For the holes in the emitter layer, the same conditions apply to the energies of the HOMOs. Examples of this may be found in: M.-J. Yang and A T. Tsutsui: “Use of poly(9-vinylcarbazole) as host material of iridium complexes in high-efficiency organic light-emitting devices” in Jpn. J. Appl. Phys. 39 (2000), Part 2, No. 8A, pp. L828–L829; R. S. Deshpande et al.: “White-light-emitting organic electroluminescent devices based on interlayer sequential energy transfer” in Appl. Phys. Lett. 75 (1999) 7, pp. 888–890; M. Hamaguchi and K. Yoshino: “Color-variable emission in multilayer polymer electroluminescent devices containing electron-blocking layer” in Jpn. J. Appl. Phys. 35 (1996), Part 1, No. 9A, pp. 4813–4818. The selection of suitable blocking layers and hence the restriction of possible emission zones is of special importance for the production of special blue OLEDs.
Reference to exciplex formation between organic emitter materials and undoped transport materials with low ionization energy are found in: K. Itano et al.: “Exciplex formation at the organic solid-state interface: yellow emission in organic light-emitting diodes using green-fluorescent tris(8-quinolinolato) aluminum and hole-transporting molecular materials with low ionization potentials” in Appl. Phys. Lett. 72 (1998) 6, pp. 636–638; T. Noda et al. “A blue-emitting organic electroluminescent device using a novel emitting amorphous molecular material, 5,5′-bis(dimesitylboryl)-2,2′-bithiophene” in Adv. Mater. 11 (1999) 4, pp. 283–285. In the latter, the use of a blocking layer for reducing this effect is proposed, although not in connection with doped transport layers. The basic dilemma, that materials with deep-lying HOMO are hard to p-dope, but materials with high-lying HOMO promote exciplex formation at the interface to the emitting layer, has not yet been recognized in the scientific literature. Accordingly, there are also no known references that propose solutions to this problem.